Title

Authors

Publication Date

8-1-2007

Degree Program

Department of Geography and Geology

Degree Type

Master of Geoscience

Abstract

This research describes the alluvial deposits and Quaternary geomorphic evolution of the Upper Green River in Kentucky by documenting the nature of flood plain sediments at two sites along the valley bottom of the Upper Green River between Mammoth Cave National Park and Green River Lake. Field methods employed included stratigraphic descriptions of bank exposures and borings obtained in flood plain surfaces, along with textural and radiocarbon analysis of sediment samples. Sediment samples from boreholes and bank exposures in two study sites (Pitman Creek confluence, -PCC- and Upper Green River Biological Preserve -UGRBP-) were collected and sieved to determine grain size distributions and stratigraphic patterns. Deposits characterized in the study area are predominantly medium to coarse silt, reflecting vertical aggradation, underlain by sands and gravels representing channel deposits associated with lateral channel migration. In collaboration with the Kentucky Geological Survey, additional boreholes were drilled at both sites using a Giddings probe. Stratigraphic sections, radiocarbon dating of buried organic material, and geomorphic observations suggest the existence of three distinctive geomorphic surfaces at PCC study site: a Lower Holocene alluvium (LHA), an Early Holocene alluvium (EHA), and a Dissected Quaternary terraces (DQT). Equivalent LHA, EHA, and DQT units were recognized downstream at the UGRBP study site. During the Pleistocene and Early Holocene epochs the Upper Green River experienced periodic entrenchment as evidenced by terraces preserved in the valley (e.g. DQT unit) followed by a renewed period of incision and aggradation stages during the Holocene reflected by alluvial units (EHA) and (LHA). Based on radiocarbon analysis of in-situ organic material, overbank sedimentation rates on the LHA surfaces of 2.5 cm/yr at the PCC site and 1.7 cm/yr at UGRBP study site are estimated for the most recent (-1750-1950 AD) deposits. Radiocarbon analysis of leaves collected from a layer at the base of an island bank exposure at the UGRBP site returned a date of 2300 ± 40 yrs BP. This layer is interpreted as remnants of older deposits incorporated into the modern river alluvium. An increase in sediment deposition rates in the historical period is consistent with increasing sediment supply due to land clearance for agriculture. Since no major Quaternary tectonic activity is reported for the region, it is reasonable to conclude that climate variability is the primary driver of Quaternary river incision and aggradation that are responsible for the valley features. The absence of paleosols in the upper 10 m of the deposits suggests rapid and continuous accumulation of sediment and the lack of stable conditions for soil development. This alluvial chronology of the Upper Green River complements the previous work on the Quaternary environmental and geomorphic evolution of the region. The contribution of new information about the textural trends and geomorphic characteristics of the Upper Green River floodplain deposits is useful to understand present-day river bank stability and sediment loading related to bank erosion. The evidence of accelerated sedimentation over the last 250 years corroborates the importance of land surface disturbance during the settlement of the region by non-native peoples. Key words: Green River, floodplain deposits, radiocarbon, Quaternary.